let y = 10;

​

function setup() {

  createCanvas(300, 300);

}

​

function draw() {

  ++x;

  background('green');

  fill('yellow');

  rect(x, y, 10, 10);

}

​

function keyPressed() {

  if (key === 'ArrowRight') {

    ++x;

  }

  if (key === 'ArrowLeft') {

    --x;

  }

  if (key === 'ArrowUp') {

    --y;

  }

  if (key === 'ArrowDown') {

    ++y;

  }

}

​